What is the primary function of MHC class I molecules in antigen presentation?

• A. Present endogenous peptide antigens to CD8+ cytotoxic T cells.
• B. Present exogenous peptide antigens to CD4+ helper T cells.
• C. Present lipid antigens to NK cells.
• D. Present peptides to B cells.

Answer: (B)

MHC class I molecules mainly present peptides from proteins made inside the cell to CD8+ T cells. Proteins in the cytosol are broken down by the proteasome into peptides, which are transported into the endoplasmic reticulum by TAP and loaded onto MHC I with help from beta-2 microglobulin, then shown on the cell surface. This allows CD8+ T cells to detect infected or abnormal cells and trigger killing or cytokine responses. In contrast, presenting exogenous peptides to CD4+ helper T cells is the job of MHC class II. Lipid antigens are presented by CD1 molecules to NKT or other T cells, not by MHC I. And B cells present peptide antigens to helper T cells via MHC II, not to other B cells.